impedance matching and impedance trace

How do you actually make impedance match? And how does impedance trace work?

Why oscilloscope terminate on 50ohm? And why most of the traces are 50ohms traces? Does it depend on how long the trace is? Or as long as it's a 50ohm trace, length doesn't matter?

What is the difference between terminating 50ohms and 1meg?

How do you actually make impedance match?

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Impedance matching circuits are designed as passive Inductor capacitor networks which transform one impedance to another impedance for particular frequencies. If you cascade such networks which match the impedance an closely spaced frequencies you can get a broad range of good matching. These circuits are generally pi and T matching circuits.

And why most of the traces are 50ohms traces?

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The 50 ohm impedance is an average impedance selected to tradeoff between two opposing effects. The line impedance should be 30 homs to maximize the conductor breakdown in a cable with an air dielectric for a given out conductor dimension. While the line impedance should be 77 ohms to minimize the attenuation in the signal going across the line. So 50 is taken as a neat average of the 2. Note Television cable lines have 75 ohms terminating impedance.

Does it depend on how long the trace is? Or as long as it's a 50ohm trace, length doesn't matter?

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If you have a termination which matches the line's caracteristic impedance the line's length does not matter much, unless your signal is so weak that the conductor losses in the line will be significant compared to the power of the signal.

What is the difference between terminating 50ohms and 1meg?

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50 ohm terminations are made in lines that require a maximum power transfer from the source generally. A 1M termination would give you maximum voltage transfer.
Hope this answered your questions.

50 Ohms is the characteristic impedance of coax cable used for scope probes.